1. Field of the Invention
This invention relates to a press forming machine for glass and, more particularly, to a press forming machine suitable for manufacturing glass products such as precision glass lenses for which high accuracy of shape is required.
2. Description of the Related Art
FIG. 4 shows a conventional press forming machine for glass. This machine is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-208243. A fixed axis 2 is fixed at a top portion of a machine frame 1 and extends downwardly therefrom. An upper die assembly 4 (fixed die) is attached to a lower end portion of the fixed axis 2 via a heat insulating block 3 made of ceramic. A screw jack 8 and a servomotor 8a serving as a drive source thereof are housed at a lower portion of the machine frame 1. A moving axis 9 has a lower end portion connected to the screw jack 8 via a load cell 8b, and extends upwardly so as to be opposite to the fixed axis 2. A lower die assembly 11 (moving die) is attached to an upper end portion of the moving axis 9 via a heat insulating block 10 made of ceramic.
The upper die assembly 4 includes a die plate 5 made of metal, a core 6 made of ceramic (or superhard alloy), and an upper die 7 which fixes the core 6 on the die plate 5 and serves as a part of the face of the die assembly 4. Similarly, the lower die assembly 11 includes a die plate 12 made of metal, a core 13 made of ceramic (or superhard alloy), and a lower die 14 which fixes the core 13 on the die plate 12 and serves as a part of the face of the die assembly 11.
An upper plate 15 is attached to the fixed axis 2. The upper plate 15 is in contact with an outer periphery of the fixed axis 2 airtightly and is slidable along the fixed axis 2. The upper plate 15 is moved up and down by a driving unit (not shown). A transparent quartz tube 16 is attached to the upper plate 15 so as to surround the upper die assembly 4 and the lower die assembly 11. A lower end portion of the quartz tube 16 is in contact with a top surface of a middle plate 1aairtightly. A forming chamber 17 is thereby formed inside the quartz tube 16.
Moreover, an outer cylinder 18 is attached to the upper plate 15 so as to surround the quartz tube 16. A lamp unit 19 (heater) which heats an interior of the forming chamber 17 is attached to an inner wall surface of the outer cylinder 18. The lamp unit 19 includes an infrared lamp 20, a reflector 21 arranged behind the infrared lamp 20, a water-cooling pipe 22 which cools the reflector 21, and air-cooling nozzles (not shown) which blow cooling air onto an outer periphery of the quartz tube 16.
Gas supply channels 23 and 24 are formed inside the fixed axis 2 and the moving axis 9, respectively. An inert gas such as N2 gas is supplied from a supply source (not shown) to the forming chamber 17 through the gas supply channels 23 and 24 and the heat insulating blocks 3 and 10 in turn. Thus, the interior of the forming chamber 17 becomes an inert atmosphere or the upper die assembly 4 and the lower die assembly 11 are cooled. In addition, a gas supply channel 25 is formed through the upper plate 15. The inert gas is directly supplied to the forming chamber 17 through the gas supply channel 25. An outlet 26 is formed through the middle plate 1a which serves as the lower portion of the forming chamber 17. The inert gas supplied to the forming chamber 17 is discharged outside the forming chamber 17 through the outlet 26.
A press forming process employing the press forming machine for glass is explained below in brief. The inert gas is supplied to the forming chamber 17 through the gas supply channels 23, 24 and 25 such that the interior of the forming chamber 17 becomes an inert atmosphere. Then, an output of the lamp unit 19 is controlled by a control unit 28 while the temperature of the lower die assembly 11 is detected by a thermocouple 27 (A thermocouple is also attached to the upper die assembly 4 though it is not shown in the figure.). Thus, the upper die assembly 4, the lower die assembly 11 and a preform 30 are heated. The speed, torque and revolution of the servomotor 8a are controlled by the control unit 28, on the basis of the detected temperature, and the moving axis 9 is thereby moved on a preprogrammed sequence.
At this time, feedback control of a pressing force applied between the upper die assembly 4 and the lower die assembly 11 is conducted such that load transmitted from the screw jack 8 to the moving axis 9 is detected by the load cell 8b provided between the screw jack 8 and the moving axis 9 and is made to correspond to the command value.
However, since the load cell 8b is attached to the lower end portion of the moving axis 9 and located outside the forming chamber 17, the press forming machine has the following problem about an accuracy of detection of the pressing force. If the pressure inside the forming chamber 17 is reduced at the press forming, the moving axis 9 is pulled up and the measured value of the load cell 8b is influenced. For this reason, an error is generated between the measured value of the load cell 8b and the pressing force which is actually applied between the upper die assembly 4 and the lower die assembly 11 and the pressing force can hardly be controlled exactly.